Vertical cavity surface emitting laser

ABSTRACT

The disclosure relates to a Vertical Cavity Surface Emitting Laser ( 100 ) comprising a first electrical contact ( 105 ), a substrate ( 110 ), a first Distributed Bragg Reflector ( 115 ), an active layer ( 120 ), a second Distributed Bragg Reflector ( 130 ) and a second electrical contact ( 135 ). The Vertical Cavity Surface Emitting Laser comprises at least two current aperture layers ( 125 ) arranged below or above the active layer ( 120 ), wherein each of the current aperture layers ( 125 ) comprises one Al y Ga (1-y) As-layer, wherein a first current aperture layer ( 125   a ) of the at least two current aperture layers ( 125 ) is arranged nearer to the active layer ( 120 ) as a second current aperture layer (125 b ) of the at least two current aperture layers ( 125 ), wherein the first current aperture layer ( 125   a ) comprises a first current aperture ( 122   a ) with a bigger size as a second current aperture ( 122   b ) of the second current aperture layer ( 125   b ). The disclosure also relates to a method of manufacturing such a VCSEL ( 100 ).

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is a Continuation 35 U.S.C. § 111 of the U.S.application Ser. No. 15/573,846 which is the U.S. National Phaseapplication under 35 U.S.C. § 371 of International Application No.PCT/EP2016/062252, filed on May 31, 2016, which claims the benefit of EPPatent Application No. EP 15171099.3, filed on Jun. 9, 2015. Theseapplications are hereby incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to a Vertical Cavity Surface Emitting Laser(VCSEL), a laser device comprising such a VCSEL and a correspondingmethod of fabricating such a VCSEL.

BACKGROUND OF THE INVENTION

State-of-the-art Vertical Cavity Surface Emitting Lasers (VCSELs) havean oxidized high-Al containing layer to form a current aperture toconfine carriers and photons. Mesa etching is needed to expose theaperture layer to the lateral oxidation process which has theside-effect that also other layers such as Distributed Bragg Reflectorlayers (DBRs) are exposed to the oxidation process. It is important thatthe parasitic oxidation rate of the DBR layers is slower than theaperture layer which limits the high-Al fraction in the DBRs to ˜90% forpractical use.

US 2010/0226402 A1 discloses a laser diode including a laminateconfiguration including a lower multilayer reflecting mirror, an activelayer and an upper multilayer reflecting mirror in order from asubstrate side, in which the laminate configuration includes a columnarmesa section including an upper part of the lower multilayer reflectingmirror, the active layer and the upper multilayer reflecting mirror, andthe lower multilayer reflecting mirror includes a plurality of pairs ofa low refractive index layer and a high refractive index layer, and aplurality of oxidation layers nonuniformly distributed in a directionrotating around a central axis of the mesa section in a region exceptfor a central region of one or more of the low refractive index layers.

EP 0 905 835 A1 discloses an independently addressable, high density,vertical cavity surface emitting laser/LED structure formed by a lateraloxidation process. The aperture of the laser structure is formed byeither lateral wet oxidation or by both selective layer intermixing andlateral wet oxidation from a semi-annular groove etched in the laserstructure.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved VCSEL.

According to a first aspect a Vertical Cavity Surface Emitting Laser isprovided. The Vertical Cavity Surface Emitting Laser comprises a firstelectrical contact, a substrate, a first Distributed Bragg Reflector, anactive layer, a second Distributed Bragg Reflector and a secondelectrical contact. The Vertical Cavity Surface Emitting Laser comprisesat least one Al_(y)Ga_((1-y))As-layer with 0.95≤y≤1 and with a thicknessof at least 40 nm, wherein the Al_(y)Ga_((1-y))As-layer is separated bymeans of at least one oxidation control layer.

The term “layer” does not exclude that the layer comprises two or moresub-layers. The VCSEL may comprise a current aperture layer forconfining a driving current supplied by means of the first and thesecond electrical contact to a defined region of the active layer. TheVCSEL may contain one, two, three, four or a multitude ofAl_(y)Ga_((1-y))As-layers. The Al_(y)Ga_((1-y))As-layer or layers may becomprised by the DBR or, for example, the current aperture layer. Thethickness of the Al_(y)Ga_((1-y))As-layer maybe at least 40 nm,preferably at least 50 nm, more preferably at least 60 nm. The thicknessof the Al_(y)Ga_((1-y))As-layer may be most preferably in the range of aquarter of the emission wavelengths of the VCSEL when driven with apredefined electrical current. The aluminum content of the

Al_(y)Ga_((1-y))As-layer may be more than 95%, preferably more than 98%,more preferably more than 99% and most preferably 100 percent. TheAl_(y)Ga_((1-y))As-layer may be separated by means of one, two, three ormore oxidation control layers. The number of oxidation control layerswithin the Al_(y)Ga_((1-y))As-layer may be limited to four or fiveoxidation control layers especially if the thickness of theAl_(y)Ga_((1-y))As-layer is in the range between 40 nm and a quarter ofthe emission wavelengths of the VCSEL. The oxidation control layer orlayers are adapted to reduce an oxidation rate of theAl_(y)Ga_((1-y))As-layer in comparison to an Al_(y)Ga_((1-y))As-layerwith the same aluminum content without oxidation control layer orlayers.

The first or the second Distributed Bragg Reflector may preferablycomprise the at least one Al_(y)Ga_((1-y))As-layer. TheAl_(y)Ga_((1-y))As-layer or layers contribute in this case to thereflectivity of the DBR.

A material of the oxidation control layer may preferably comprise orconsist of Al_(x)Ga_((1-x))As with 0≤x≤0.9. The range of the aluminumcontent may preferably be between 0.2 and 0.7, more preferably between0.4 and 0.6. Oxidation control layers of pure gallium arsenide may havethe disadvantage that absorption increases above an emission wavelengthof around 800 nm if relatively thick oxidation control layers are used.The effect is negligible in case of thin oxidation control layers(thickness of around lnm).

The Vertical Cavity Surface Emitting Laser comprises preferably at leastone Al_(y)Ga_((1-y))As-layer with an aluminum content y>0.99. The atleast one Al_(y)Ga_((1-y))As-layer is preferably separated by means ofat least two oxidation control layers, more preferably exactly 2 to 5oxidation control layers. The material of the oxidation control layercomprises preferably Al_(x)Ga_((1-x))As with 0.4≤x≤0.6, more preferablywith an aluminum content of around 50%. Each low refractive index layerof at least the first (bottom) DBR preferably comprises or consists of aAl_(y)Ga_((1-y))As-layer with an aluminum content y>0.99 and comprisingat least one oxidation control layers. It may be further advantageousthat the second DBR and one or more current aperture layer comprises orconsists of a Al_(y)Ga_((1-y))As-layer with an aluminum content y>0.99and comprising at least one oxidation control layers. The currentaperture layer may have a thickness of half of the emission wavelengthsor an integer multiple thereof.

The at least one oxidation control layer has a thickness between 0.7 nmand 3 nm, preferably between 0.8 nm and 2 nm, more preferably between0.9 nm and 1.5 mm. The thickness of the at least one oxidation controllayer (119, 125 b) may comprise between 3% and 10% of a total thicknessof the Al_(y)Ga_((1-y))As-layer. The total thickness of theAl_(y)Ga_((1-y))As-layer is determined by the entire thickness of allsub-layers of the Al_(y)Ga_((1-y))As-layer which are separated by theoxidation control layer or layers and the thickness of the oxidationcontrol layer or layers.

The current aperture layer may comprise the at least oneAl_(y)Ga_((1-y))As-layer. The current aperture layer may have athickness of half of the emission wavelength of the VCSEL or an integermultiple thereof. The current aperture layer would in this case have noeffect with respect to reflectivity of an adjacent DBR. Preferably, thecurrent aperture layer may have thickness of a quarter of the emissionwavelength of the VCSEL or an uneven multiple thereof such that thecurrent aperture layer may contribute to the reflectivity of an adjacentDBR or may even be part of the DBR.

The Vertical Cavity Surface Emitting Laser comprises a first electricalcontact, a substrate, a first Distributed Bragg Reflector, an activelayer, a second Distributed Bragg Reflector and a second electricalcontact. The Vertical Cavity Surface Emitting Laser may comprise atleast two current aperture layers, wherein the current aperture layersare arranged below or above the active layer. Both of the at least twocurrent aperture layers may be preferably be arranged below or above theactive layer. Alternatively it may be possible that one of the at leasttwo current aperture layers may be arranged below the active layer andthe other one of the at least two current aperture layers may bepreferably be arranged above the active layer. A first current aperturelayer of the at least two current aperture layers may be arranged nearerto the active layer as a second current aperture layer of the at leasttwo current aperture layers. Nearer means in this respect a nearerdistance between the layers perpendicular to the layers. It may bepreferred that the first current aperture layer of the at least twocurrent aperture layers may be arranged between the active layer and thesecond current aperture layer of the at least two current aperturelayers. The first current aperture layer may comprise a first currentaperture with a bigger size as a second current aperture of the secondcurrent aperture layer. The size of the current apertures is given bythe non-oxidized parts of the current aperture layers. The currentaperture may have a circular shape such that the size of the currentaperture can be defined by means of the diameter. The shape of thecurrent aperture may alternatively be elliptical, rectangular,triangular and the like. The shape of the current aperture is mainlydetermined by the shape of the mesa of the VCSEL and the oxidationprocess. The current apertures comprise a common symmetry axis such thatin case of circular current apertures the centers of the circles arearranged along this common symmetry axis. Each of the at least twocurrent aperture layers may preferably comprise aAl_(y)Ga_((1-y))As-layer with one or more oxidation control layer orlayers. The thickness of the Al_(y)Ga_((1-y))As-layers may be less than40 nm, for example 30 nm or even 20 nm. The Al_(y)Ga_((1-y))As-layersmay simplify manufacturing or processing of current apertures withdifferent sizes such that the size of the apertures can be manufacturedin a precise way. Alternatively, oxidation of AlGaAs layers may becontrolled by providing defined variations of the aluminum contentwithin the AlGaAs layers (layers with graded aluminum content) ordifferent Al concentrations of the AlGaAs layers. The at least twocurrent aperture layers may in this case also comprise AlGaAs layerswith an average aluminum concentration or aluminum concentration of lessthan 95%. The aluminum concentration within the AlGaAs layers needs tobe controlled in a very precise way in order to enable sufficientcontrol of the oxidation width of the different current aperture layerssuch that Al_(y)Ga_((1-y))As-layers with oxidation control layers may bepreferred. The first and the second current aperture layer are arrangedsuch that during operation of the VCSEL high current densities at theedge of the first current aperture are avoided or at least limited suchthat high reliability and lifetime of the VCSEL is enabled. The firstand the second current aperture layer may preferably be arranged in thelayer arrangement of the first or the second DBR.

The first current aperture layer with the first current aperture maypreferably be attached to the upper side or lower side of the activelayer, or to phrase it differently, that near to the active layer suchthat a lateral spread of the charge carriers is avoided. The secondcurrent aperture layer with the second current aperture is arranged suchthat current densities at the edge of the first current aperture areless than 100 kA/cm² during operation of the VCSEL. Limitation of thecurrent at the edge of the first current aperture (current peaking)increases reliability and lifetime of the VCSEL and may in additionavoid support of higher-order laser modes which may be unwantedespecially for single mode VCSELs.

The second current aperture or more general the current aperture withthe smallest size may preferably be arranged at a distance correspondingto an integer multiple of halve of the emission wavelength of the VCSEL,more preferably at a distance corresponding to an even multiple of halveof the emission wavelength of the VCSEL and most preferably at adistance of six times halve of the emission wavelength of the VCSEL tothe active layer. The distance between the active layer which is a highrefractive layer in comparison to the low refractive layers of the DBRsand the second current aperture is taken from a node of the standingwave pattern within the active layer (comprising supporting layers) atthe side of the active layer next to the layer with the second currentaperture and a node of the standing wave pattern within the secondcurrent aperture layer comprising the second current aperture. Theoxidation profile of the current aperture with the smallest size may betapered in order to avoid optical guiding.

The Vertical Cavity Surface Emitting Laser may preferably bemanufactured according following method. The method comprises the stepsof:

-   -   providing a first electrical contact,    -   providing a substrate,    -   providing a first Distributed Bragg Reflector,    -   providing an active layer,    -   providing a second Distributed Bragg Reflector,    -   providing a second electrical contact,    -   providing at least two current aperture layers, wherein the at        least two current aperture layers are arranged below or above        the active layer,    -   arranging a first current aperture layer of the at least two        current aperture layers nearer to the active layer as a second        current aperture layer of the at least two current aperture        layers,    -   providing a first current aperture in the first current aperture        layer,    -   providing a second current aperture in the second current        aperture layer with a smaller size as the first current        aperture.

The method steps need not necessarily be performed in the order givenabove. The substrate may, for example, be provided in a first step andthe first electrical contact in a second step. Providing the first andthe second current aperture may comprise the steps of providing thefirst and the second current aperture layer and oxidizing these layersin a subsequent step. The first and the second current aperture layersmay be provided by alternately depositing sublayers and oxidationcontrol layers as described above and below. Number of oxidation controllayers and distance between the oxidation control layers may be used inorder to control the oxidation width and thus the size of the currentapertures. The first and the second current aperture layers mayalternatively be provided by depositing layers with smoothly varyingaluminum content or different aluminum concentration within layers asdescribed above. The variation of their aluminum content or the aluminumcontent may be adapted in the layers to the intendant oxidation width ofthe current aperture layers.

The oxidation process may alternatively be performed by means of currentaperture layers with the same aluminum content. The oxidation width ofthe different current apertures may be controlled by subsequentlyetching an oxidation opening to the respective oxidation control layer.It may also be possible to combine sequential etching of the currentapertures with different aluminum content and/or oxidation controllayers. The difference between the size of the first current apertureand the second current aperture is preferably between 1 μm and 6 μm indiameter taking a circular aperture as reference.

All preferred embodiments described above and in the following may alsobe comprised in a VCSEL comprising the at least first and second currentaperture layers wherein the first current aperture has a bigger size.

The VCSEL may comprise three, four, five or more current aperture layerswith current apertures. The size of a current aperture of at least oneof the current aperture layers being arranged on the side of the firstcurrent aperture layer next to the active layer is smaller than the sizeof the first current aperture. The size of two or more of the currentapertures may be the same. Alternatively, the size of all currentapertures may be different, wherein the size of the current aperturesdecreases in a direction perpendicular to the active layer, wherein thefirst current aperture has the biggest size. The current aperture layersmay be arranged equidistantly such that the distance between twoadjacent current aperture layers perpendicular to the direction of theactive layer is the same for all current aperture layers. Alternatively,it may be possible that the distance between the current aperture layersvaries. The first or second DBR may, for example, comprise a first lowrefractive index layer comprising the first current aperture layer withthe first current aperture. The fourth low refractive index layer maycomprise the second current aperture layer the second current apertureand the fifth low refractive index layer may comprise the third currentaperture layer with a third current aperture. The size of the secondcurrent aperture may be smaller than the size of the third currentaperture.

The Vertical Cavity Surface Emitting Laser may comprise at least oneAl_(y)Ga_((1-y))As-layer or one, two, three or moreAl_(y)Ga_((1-y))As-layers which comprise a tapered oxidation profile.The at least one Al_(y)Ga_((1-y))As-layer with the tapered oxidationprofile may comprise preferably at least two oxidation control layers.The at least two oxidation control layers separate the at least oneAl_(y)Ga_((1-y))As-layer in at least three sub-layers and wherein atleast one of the three sub-layers has a different thickness as the othersub-layers. The sub-layer with the different thickness is preferablythicker in comparison to the other sub-layers. The thicker sub-layeroxidizes faster as the adjacent sub-layer such that the taperedoxidation profile is built during the oxidation process. The tapperedoxidation profile comprises a waistline meaning the smallest diameterwithin the Al_(y)Ga_((1-y))As-layer which is not oxidized during theoxidation process. The waistline of the tapered oxidation profile ispreferably arranged in a range of a node of a standing wave pattern ofthe Vertical Cavity Surface Emitting Laser when driven at a predefinedelectrical driving current. In the range of the node means that thewaistline is arranged much nearer to the node than the maximum of thestanding wave pattern. The distance between the node and the waistlineis preferably less than 35 nm, more preferably less than 25 nm.Arranging the waistline of the tappered oxidation profile in range ofthe node of the standing wave pattern may have the advantage that strongguiding of the standing wave pattern within the thickAl_(y)Ga_((1-y))As-layer is avoided or at least reduced. Such guiding isusually avoided or limited by using thin current aperture layers with athickness of around 30 nm or less.

The first or the second Distributed Bragg Reflector may comprisemultitude Al_(y)Ga_((1-y))As-layers, wherein theAl_(y)Ga_((1-y))As-layers are separated by means of at least oneoxidation control layer. The Al_(y)Ga_((1-y))As-layers may comprise amaximum of 3 oxidation control layers. The Al_(y)Ga_((1-y))As-layers maybe arranged to reduce the thermal resistance of the Vertical CavitySurface Emitting Laser (100) to a cooling structure when mounted on thecooling structure. The DBR comprising the Al_(y)Ga_((1-y))As-layers maybe the top or bottom DBR depending on the arrangement of VCSEL andcooling structure. Most common arrangement will be the bottom DBR incase of a top emitting VCSEL. The high aluminum content of theAl_(y)Ga_((1-y))As-layers result in a high thermal conductivity. Thealuminum content may thus preferably be as high as possible, forexample, hundred percent. The Al_(y)Ga_((1-y))As-layers are in this caseAlAs-layers.

The high aluminum content of the Al_(y)Ga_((1-y))As-layers may furtherbe used to reduce the parasitic capacitance of the VCSEL. The first orthe second Distributed Bragg Reflector may thus comprise a multitude ofAl_(y)Ga_((1-y))As-layers. The Al_(y)Ga_((1-y))As-layers are separatedby means of at least one oxidation control layer and preferably amaximum of 3 oxidation control layers. The DBR comprising theAl_(y)Ga_((1-y))As-layers may be the top or bottom DBR depending on thearrangement of VCSEL. Most common arrangement will be the top DBR incase of a top emitting VCSEL.

The first and the second Distributed Bragg Reflector comprise amultitude of high refractive index layers and a multitude of lowrefractive index layers, wherein the low refractive index layerscomprise said Al_(y)Ga_((1-y))As-layers or are saidAl_(y)Ga_((1-y))As-layers. The Al_(y)Ga_((1-y))As-layers are separatedby means of at least one oxidation control layer and preferably amaximum of 3 oxidation control layers.

According to a second aspect a laser device is provided. The laserdevice comprises at least one Vertical Cavity Surface Emitting Laseraccording to any embodiments described above and an electrical drivingcircuit for electrically driving the Vertical Cavity Surface EmittingLaser. The laser device optionally may further comprise an electricalpower supply like, for example, a battery or rechargeable batteryarrangement. The laser device may be coupled to an optical sensordevice, optical datacom device or the like.

According to a third aspect a method of fabricating a Vertical CavitySurface Emitting Laser is provided. The method comprises the steps of:

-   -   providing a first electrical contact,    -   providing a substrate,    -   providing a first Distributed Bragg Reflector,    -   providing an active layer,    -   providing a second Distributed Bragg Reflector,    -   providing a second electrical contact,    -   providing at least one Al_(y)Ga_((1-y))As-layer with 0.95≤y≤1        and with a thickness of at least 40 nm, wherein the        Al_(y)Ga_((1-y))As-layer is separated by means of at least one        oxidation control layer.

The method steps need not necessarily be performed in the order givenabove. The substrate may, for example, be provided in a first step andthe first electrical contact in a second step. The at least oneAl_(y)Ga_((1-y))As-layer may be provided within the step of providingthe first and/or second DBR. The method may optionally comprise anadditional step of providing a current aperture layer which may be theat least one Al_(y)Ga_((1-y))As-layer. According to a fourth aspect amethod of fabricating a laser device is provided. The method comprisesthe steps of:

-   -   providing a VCSEL as described above,    -   providing an electrical driving circuit, and optionally    -   providing an electrical power supply.

Further advantageous embodiments are defined below.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

The invention will now be described, by way of example, based onembodiments with reference to the accompanying drawings.

In the drawings:

FIG. 1 shows a principal sketch of a first VCSEL

FIG. 2 shows a principal sketch of a current aperture layer

FIG. 3 shows a principal sketch of a second VCSEL

FIG. 4 shows a principal sketch of the first DBR of the second VCSEL

FIG. 5 shows a principal sketch of a low refractive index layer of thefirst DBR

FIG. 6 shows a principal sketch of a layer structure of a third VCSEL

FIG. 7 shows a principal sketch of a layer structure of a fourth VCSEL

FIG. 8 shows a principal sketch of a layer structure of a fifth VCSEL

FIG. 9 shows a principal sketch of a layer structure of a sixth VCSEL

FIG. 10 shows a principal sketch of a layer structure of a seventh VCSEL

FIG. 11 shows a principal sketch of a eighth VCSEL

FIG. 12 shows a principal sketch of an oxidation profile

FIG. 13 shows a principal sketch of a standing wave pattern

FIG. 14 shows a principal sketch of a layer structure of an ninth VCSEL

FIG. 15 shows a principal sketch of a laser device

FIG. 16 shows a principal sketch of a process flow of a method offabricating a VCSEL

In the Figures, like numbers refer to like objects throughout. Objectsin the Figures are not necessarily drawn to scale.

DETAILED DESCRIPTION OF EMBODIMENTS

Various embodiments of the invention will now be described by means ofthe

Figures.

FIG. 1 shows a principal sketch of a first VCSEL 100. The first VCSEL100is a top emitting VCSEL100 emitting laser light in a direction away froma substrate 110. On the bottom side of the substrate 110 a firstelectrical contact 105 is provided. On the top side of the substrate isa first DBR 115 provided comprising 30 pairs of layers with a first anda second refractive index. The pairs of layers of the first DBR 115comprise AlGaAs layers with high and low aluminum content. The aluminumcontent may in extreme cases and depending on the emissions wavelengthof the VCSEL 100 be 0% for the high refractive index material and up to100% for the low refractive index material. The thickness of the layersis adapted to the emission wavelength of the VCSEL 100 (quarterwavelength thickness) in order to provide the requested reflectivity ofmore than 99.9%. On top of the first DBR 115 is an active layer 120provided. The active layer 120 comprises a quantum well structure forlight generation. An n-current injection layer (not shown) may bearranged between the first DBR 115 and the active layer 120. The VCSEL100 further comprises a current aperture layer 125 comprising orconsisting of an Al_(0.98)Ga_(0.02)As-layer with a thickness of around70 nm which is in the range of a quarter of the emission wavelength ofthe VCSEL 100 of 850 nm. The Al_(0.98)Ga_(0.02)As-layer comprises fiveoxidation control layers 125 b comprising or consisting of aAl_(0.2)Ga_(0.8)As-layer with a thickness of 1 nm. The second DBR 130comprises 15 pairs of layers which comprise AlGaAs layers with high andlow aluminum content similar or even identical to the AlGaAs layers ofthe first DBR. The thickness of the pair of layers is adapted to theemission wavelength of the VCSEL100 in order to provide the requestedreflectivity of around 95%. A ring shaped second electrical contact 135is electrically contacted to the electrically conductive second DBR 130.The VCSEL 100 emits laser light in the direction of the arrow via thesecond DBR 130. The current aperture layer 125 is arranged above theactive layer 120.

FIG. 2 shows a principal sketch of the current aperture layer 125 withtwo Al_(0.98)Ga_(0.02)As-sub-layers 125 a which are separated by theoxidation control layer 125 b. The thickness of theAl_(0.98)Ga_(0.02)As-sub-layers 125 a is different in order to enable anoxidation profile within the current aperture layer 125. The oxidationcontrol layer 125 b enables improved control of the oxidation of thecurrent aperture layer 125 during manufacturing of the VCSEL 100.

FIG. 3 shows a principal sketch of a second VCSEL 100. The second VCSEL100 is a top emitting VCSEL 100 emitting laser light in a direction awayfrom a substrate 110. On the top side of the substrate is a first DBR115 provided. The active layer 120 is provided on top of the first DBR115. The current aperture layer 125 with current aperture 122 isprovided on top of the active layer 120. A second DBR 130 is provided ontop of the current aperture layer 125. The current aperture layer 125comprises of consists of AlAs. The thickness of the current aperturelayer 125 which comprises two oxidation control layers 125 b is 40 nm. Asecond electrical contact 135 is electrically contacted to theelectrically conductive second DBR 130. The VCSEL 100 emits laser lightin the direction of the arrow via the second DBR 130. The VCSEL 100 maycomprise further layers as, for example, buffer layers which are notshown. The VCSEL 100 is mounted with the substrate 110 on a coolingstructure 150. The first electrical contact 105 is arranged as anintracavity contact on a conductive layer arranged within the first DBR115.

FIG. 4 shows a principal sketch of the first DBR 115 of the second VCSEL100. The first DBR 115 comprises alternating sequence of 40 highrefractive index layers 116 and low refractive index layers 117. Thehigh refractive index layer 116 comprises or consists of theAl_(0.05)Ga_(0.95)As and low refractive index layer 117 comprises orconsists of AlAs with two oxidation control layers 119 comprising orconsisting of Al_(0.5)Ga_(0.5)As with the thickness of 1 nm. FIG. 5shows a principal sketch of a low refractive index layer 117 of thefirst DBR 115. The low refractive index layer 117 is separated by meansof two oxidation control layers 119 such that the three AlAs-sub-layers118 have the same thickness. The high thermal conductivity of theAlAs-layers reduces thermal resistance between the active layer 120 andthe cooling structure 150. The oxidation control layers 119 enableimproved control of the oxidation of the current aperture layer 125during manufacturing of the VCSEL 100. The oxidation control layers 119avoid faster oxidation of the low refractive index layers 117 incomparison to oxidation of the current aperture layer 125.

FIG. 6 shows a principal sketch of the layer structure of a third VCSEL100. The layer structure is shown around the active layer 120. Thevertical axis 200 shows the AlAs-content of the layers. Horizontal axis210 shows the direction across VCSEL 100 along the emission directionwith the first DBR 115 at the left and the second DBR 130 at the right.Four layers of the first DBR 115 are shown. The first low refractiveindex layer 117 at the left comprises or consists of AlAs with threeoxidation control layers 119 comprising or consisting ofAl_(0.5)Ga_(0.5)As. The second low refractive index layer 117 at theleft comprises or consists of AlAs with two oxidation control layers 119comprising or consisting of Al_(0.5)Ga_(0.5)As. The current aperturelayer 125 a rranged on the left of the active layer 120 comprises orconsists of AlAs without any oxidation control layers. The currentaperture layer 125 has a thickness of 30 nm. Four layers of the firstDBR 115 are shown. The second low refractive index layer 117 at theright site active layer 120 comprises or consists of AlAs with oneoxidation control layer 119 comprising or consisting ofAl_(0.5)Ga_(0.5)As. The second and the third low refractive index layer117 at the right side of the active layer 120 comprises or consists ofAlAs with two oxidation control layers 119 comprising or consisting ofAl_(0.5)Ga_(0.5)As. Oxidation control layers 119 have a thickness of 1nm wherein the total thickness of the low refractive index layers 117 isaround 70 nm.

Experiments have shown that an AlAs-layer with a thickness of 30 nm andwithout any oxidation control layer 119 arranged in a stack as shown inFIG. 6 oxidizes at 370° C. within around 72 minutes 38 μm. An AlAs-layerwith a thickness of 70 nm which may be used as a low refractive indexlayer 117 within a DBR of a VCSEL with emission wavelengths of 850 nmoxidizes 45 μm under the same conditions as the 30 nm layer. It is thusnot possible to use such 70 nm AlAs-layer as low refractive index layer117 in a DBR because oxidation is faster in comparison to oxidation of a30 nm current aperture layer 125. In comparison 70 nm an AlAs-layercomprising one oxidation control layer 119 with a thickness of 1 nm andcomprising or consisting of Al_(0.5)Ga_(0.5)As oxidizes 28.6 nm, and a70 nm AlAs-layer comprising two oxidation control layer 119 with athickness of 1 nm and comprising or consisting of Al_(0.5)Ga_(0.5)Asoxidizes only 10 nm oxidized under the same conditions. The oxidationcontrol layers 119 separate the AlAs-layer in the experiment inAlAs-sub-layer 118 of the same thickness. The oxidation control layers119 enable to use pure AlAs as low refractive index layer 117. Theoxidation control layers 119 enable depending on the number andpositioning of oxidation control layers 119 a superior control of theoxidation width.

FIG. 7 shows a principal sketch of a layer structure of a fourth VCSEL100. The fourth VCSEL100 is a top emitting VCSEL 100 emitting laserlight in a direction away from substrate 110. On the bottom side of thesubstrate 110 a first electrical contact 105 is provided. On the topside of the substrate is a first DBR 115 provided comprising 30 pairs oflayers with a first and a second refractive index. The pairs of layersof the first DBR 115 comprise AlGaAs layers with high and low aluminumcontent. The aluminum content may in extreme cases and depending on theemissions wavelength of the VCSEL 100 be 0% for the high refractiveindex material and up to 100% for the low refractive index material. Thethickness of the layers is adapted to the emission wavelength of theVCSEL 100 (quarter wavelength thickness) in order to provide therequested reflectivity of more than 99.9%. On top of the first DBR 115is an active layer 120 provided. The active layer 120 comprises aquantum well structure for light generation. An n-current injectionlayer (not shown) may be arranged between the first DBR 115 and theactive layer 120. The VCSEL 100 further comprises a first currentaperture layer 125 a comprising or consisting of AlGaAs with an averagecomposition of Al_(0.90)Ga_(0.1)As-layer with a thickness of around 30nm. It may be preferred to have a higher aluminum content of at least95% in order to enable high oxidation rates and therefore reducedoxidation times. The first current aperture layer 125 a comprises afirst current aperture 122 a which is arranged on top the active layer120 and is arranged on the upper surface of the active layer 120 inorder to enable a good current confinement. The second DBR 130 comprises15 pairs of layers which comprise AlGaAs layers with high and lowaluminum content similar or even identical to the AlGaAs layers of thefirst DBR. The thickness of the pair of layers is adapted to theemission wavelength of the VCSEL100 in order to provide the requestedreflectivity of around 95%. A second current aperture layer 125 b isarranged within the layer stack of the first DBR. The second currentaperture layer 125 b is one of the low refractive index layers 117 ofthe first DBR and has a thickness of around 70 nm which corresponds to aquarter of the emission wavelengths of the VCSEL 100 of around 850 nm.The second current aperture layer 125 b comprises a second currentaperture 122 b with a smaller diameter as the first current aperture 122a of the first current aperture layer 125 a. The diameters of the firstcurrent aperture 122 a and the second current aperture 122 b and thedistance between the first current aperture layer 125 a and the secondcurrent aperture layer 125 b perpendicular to the layers of the VCSEL100 as well as the distance between the first current aperture layer 125a and the active layer are chosen such that a good current confinementis enabled but high current densities of, for example, more than 100kA/cm² at the edge of the first current aperture 122 a are avoided.Furthermore, the distance between the active layer 120 and the secondcurrent aperture 122 b with the smallest aperture size is bigger incomparison to conventional VCSEL 100 such that low optical guiding isenabled. Usually, low optical guiding is preferred because it supportsnarrow beam divergence, higher single mode power, less spectral widthVCSELs, high brightness designs, etc.. Providing the current aperturelayers 125 within the first or the second DBR may have the advantagethat the current aperture layers 125 contribute to reflectivity of thefirst or the second DBR. In addition manufacturing of such currentaperture layers 125 may be simplified because the aluminum profilematches the demands of DBR design. A ring shaped second electricalcontact 135 is electrically contacted to the electrically conductivesecond DBR 130. The VCSEL 100 emits laser light in the direction of thearrow via the second DBR 130. The current aperture layers 125 a rearranged above the active layer 120.

FIG. 8 shows a principal sketch of a part of a layer structure of afifth VCSEL 100. The VCSEL 100 is a top emitting VCSEL 100. The secondDBR 130 of which a part of the high refractive and low refractive indexlayers 116, 117 are shown is deposited on top of the active layer 120.The second DBR 130 comprises three low refractive index layers 117 whichare arranged as current aperture layers 125. The first current aperturelayer 125 a is the second low refractive index layer 117 of the secondDBR 130 in a direction away from active layer 120. The second currentaperture layer 125 b is the third low refractive index layer 117 and thethird current aperture layer 125 c is the fourth low refractive indexlayer 117 of the second DBR 130. The distance between the currentaperture layers 125 is equal. The first current aperture 122 a which isarranged next to the active layer 120 has the biggest size meaning inthe case of a circular aperture the biggest diameter. The second currentaperture 122 b of the second current aperture layer 125 b has thesmallest size and is arranged between the first and the second currentaperture layers 125 a, 125 c. The current aperture of the third currentaperture layer 125 c has a size which is in between of the sizes of thefirst current aperture 125 a and the second current aperture 122 b.

FIG. 9 shows a principal sketch of a part of a layer structure of asixth VCSEL. The VCSEL 100 is a top emitting VCSEL 100. The second DBR130 of which a part of the high refractive and low refractive indexlayers 116, 117 are shown is deposited on top of active layer 120. Thesecond DBR 130 comprises three low refractive index layers 117 which arearranged as current aperture layers 125. The first current aperturelayer 125 a is the second low refractive index layer 117 of the secondDBR 130 in a direction away from the active layer 120. The secondcurrent aperture layer 125 b is the fourth low refractive index layer117 and the third current aperture layer 125 c is the third lowrefractive index layer 117 of the second DBR 130. The distance betweenthe current aperture layers 125 is equal. The first current aperture 122a which is arranged next to the active layer 120 has the biggest sizemeaning in the case of a circular aperture the biggest diameter. Thecurrent aperture of the third current aperture layer 125 c has the samesize as the first current aperture 125 a. The second current aperture122 b of the second current aperture layer 125 b has the smallest sizeand is arranged above the first and the third current aperture layers125 a, 125 c in the direction away from the active layer 120. Thecurrent apertures 125 a, 125 b and 125 c may in an alternativeembodiment be arranged in the first DBR 115 below the active layer 120.

FIG. 10 shows a principal sketch of a layer structure of a seventhVCSEL. The layer structure is very similar to the layer structure of thesixth VCSEL shown in FIG. 9. . The first current aperture layer 125 awith the first current aperture 122 a is in this case the first lowrefractive index layer 117 of the first DBR 115 below the active layer120. The active layer 120 is arranged in this case between the firstcurrent aperture layer 125 a and the second current aperture layer 125 bwhich is arranged in the third low refractive index layer 117 of thesecond DBR 130.

FIG. 11 shows a principal sketch of a eighth VCSEL 100. The seventhVCSEL 100 is a bottom emitting VCSEL 100 emitting laser light in adirection of a substrate 110. The substrate 110 is removed at the areaat which the laser light is emitted. The direction of light emission isindicated by an arrow. On the bottom side of the substrate 110 a firstelectrical contact 105 is provided around the removed part of thesubstrate 110. On the top side of the substrate is a first DBR 115provided with a reflectivity of around 95% in order to enable laserlight emission via the first DBR 115. A current aperture layer 125 isprovided on top of the first DBR 115. The active layer 120 is providedon top of the current aperture layer 125. A second DBR 130 is providedon top of the active layer 120 with a reflectivity of more than 99.9%. Asecond electrical contact 135 is electrically connected to theelectrically conductive second DBR 130.

FIG. 12 shows a principal sketch of an oxidation profile 126 with in thecurrent aperture layer 125 of the seventh VCSEL 100. The currentaperture layer 125 comprises four Al_(0.99)Ga_(0.01)As-sub-layers 125 awhich are separated by three the oxidation control layers 125 bcomprising or consisting of Al_(0.7)Ga_(0.3)As with a thickness of 0.8nm. The thickness of the Al_(0.99)Ga_(0.02)As-sub-layers 125 a isdifferent in order to enable an oxidation profile within the currentaperture layer 125. The upper Al_(0.99)Ga_(0.02)As-sub-layer 125 a nextto the active layer 120 (see FIG. 7) has a thickness of around 35 nm,the next Al_(0.99)Ga_(0.02)As-sub-layer 125 a below thickness of 20 nm,and the other two Al_(0.99)Ga_(0.02)As-sub-layers 125 a have a thicknessof 7 nm. The total thickness of the current aperture layer 125 is in therange of a quarter of the emission wavelength of 850 nm of the VCSEL 100shown in FIG. 7. The oxidation control layer 125 therefore contributesto the reflection of first DBR 115. The different thickness of theAl_(0.99)Ga_(0.02)As-sub-layers 125 a result in an oxidation profile 126with a waistline 127 in the upper Al_(0.99)Ga_(0.02)As-sub-layer 125 anear to the active layer 120. The waistline 127 is arranged in a rangeof a node of the standing wave pattern 250 of the VCSEL 100 when drivenat a predefined current as depicted in FIG. 13. The oxidation controllayers 125 b separating the current aperture layer 125 in sub-layers ofdifferent thickness enables control of the oxidation of the currentaperture layer 125 during manufacturing of the VCSEL 100 such that adefined oxidation profile is built. Alternative methods use, forexample, a predetermined variation of the aluminum content across thecurrent aperture layer 125. Experiments have shown that defined controlof the aluminum content within the current aperture layer across thewhole wafer or even several wafers is difficult during the manufacturingprocess and reduces production yield substantially. Therefore, thincurrent aperture layers with a thickness of 30 nm or less are used inconventional VCSEL in order to avoid strong guiding of the laser light.

Experiments with a AlAs current aperture layer 125 with a thickness ofaround 70 nm comprising two oxidation control layers 125 a consisting ofAl_(0.5)Ga_(0.5)As with a thickness of 1 nm have been made. The upperAlAs sub-layer 125 a was 2 nm thicker than the other two AlAs sub-layers125 a. The spectrum of the emitted laser light confirmed a taperedoxidation profile 126 within the current aperture layer 125 with awaistline 127 of the standing wave pattern.

FIG. 14 shows a principal sketch of a part of a layer structure of anninth VCSEL 100. The VCSEL 100 is in this case a bottom emitting VCSEL100 similar as described in FIG. 10. Active layer 120 is deposited ontop of the first DBR 115 of which a part of the low and high refractivelayers 116, 117 are shown. The first DBR 115 comprises four lowrefractive index layers 117 which are arranged as current aperturelayers 125. The first current aperture layer 125 a is the uppermost lowrefractive index layer 117 of the first DBR 115 which is arrangeddirectly below the active layer 120. The second current aperture layer125 b is the fifth low refractive index layer 117 starting with thefirst current aperture 125 a layer as the first low refractive indexlayer 117. The third current aperture layer 125 c is the third lowrefractive index layer 117 of the second DBR 130 and the fourth currentaperture layer 125 d if the fourth low refractive index layer 117 of thesecond DBR 130. The distance between the first current aperture layer125 a and the following current aperture layer (third current aperturelayer 125 c) is twice as big as the distance between the third, thefourth and the second current aperture layers 125 c, 125 d , 125 b. Thefirst current aperture 122 a which is arranged next to the active layer120 has the biggest size meaning the biggest area for passing electricalcurrent. The subsequent current apertures in a direction away from theactive layer subsequently decrease in size. The second current aperture122 b of the second current aperture layer 125 b has the smallest sizeand is arranged below the first, the third and the fourth currentaperture layers 125 a, 125 c in the direction away from the active layer120.

FIG. 14 shows a principal sketch of laser device 300 comprising amultitude of VCSELs 100 with DBRs comprising low refractive index layers117 consisting of Aluminum Arsenide (AlAs). The VCSELs 100 are arrangedin a laser array 330. The configuration of a single VCSEL 100 isessentially the same as the configuration of the second VCSEL 100 shownin FIG. 3. The laser device 300 further comprises an electrical drivingcircuit 310 and an electrical power supply 320 which is a rechargeablebattery. The electrical driving circuit 310 is arranged to supplyelectrical power provided by means of electrical power supply 320 in adefined way to laser array 330.

FIG. 15 shows a principal sketch of a process flow of a method offabricating a VCSEL100 according to the present invention. A firstelectrical contact is provided in step 410. The first electrical contactis attached to a bottom side of a GaAs substrate which is provided instep 420. A first DBR 115 is provided on a top side of the substrate instep 430 and an active layer 120 is provided in subsequent step 440 ontop of the first DBR 115. On top of the active layer 120 is a second DBRprovided in step 450. A second electrical contact is provided forelectrically contacting the VCSEL100 step 460. At least oneAl_(y)Ga_((1-y))As-layer with 0.95≤y≤1 and with a thickness of at least40 nm is provided in step 470. The manufacturing step of providing theAl_(y)Ga_((1-y))As-layer with 0.95≤y≤1 comprises depositing a firstAl_(y)Ga_((1-y))As-sub-layer 118, 125 a, depositing at least oneoxidation control layer 119, 125 b and depositing at least a secondAl_(y)Ga_((1-y))As-sub-layer 118, 125 a on top of the at least oneoxidation control layer 119, 125 b. The method may further comprise thestep of providing a current aperture layer 125.

The layers of the first DBR 115, the second DBR 130, the active layer120, the current aperture layer 125 a nd any other layer as currentinjection layers, buffer layers and the like may be deposited byepitaxial methods like MOCVD.

It is the intention of the present invention to provide a VCSEL 100which can be easily processed in a reliable way by enabling definedoxidation of 1, 2, 3 or more current aperture layers 125 a nd usingAl_(y)Ga_((1-y))As with a minimum aluminum content of 95% as lowrefractive index layers 117. The invention enables to provide a definedoxidation profile within a thick (e.g. quarter wavelength) currentaperture layer 125 which is adapted to interact with a standing wavepattern of the VCSEL 100 in an optimized way. The high aluminum contentof the low refractive index layers 117 which may be used within one orboth DBRs of the VCSEL 100 enables high thermal conductivity and reducedparasitic capacitance. Lifetime and switching behavior of the VCSEL 100may be improved without reliability and yield problems which are usuallycaused by thick (quarter wavelength) Al_(y)Ga_((1-y))As-layers with aminimum aluminum content of 95% and especially AlAs-layers becauseoxidation of such layers cannot sufficiently controlled across a waverduring the manufacturing process.

While the invention has been illustrated and described in detail in thedrawings and the foregoing description, such illustration anddescription are to be considered illustrative or exemplary and notrestrictive.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the art and which may be usedinstead of or in addition to features already described herein.

Variations to the disclosed embodiments can be understood and effectedby those skilled in the art, from a study of the drawings, thedisclosure and the appended claims. In the claims, the word “comprising”does not exclude other elements or steps, and the indefinite article “a”or “an” does not exclude a plurality of elements or steps. The mere factthat certain measures are recited in mutually different dependent claimsdoes not indicate that a combination of these measures cannot be used toadvantage.

Any reference signs in the claims should not be construed as limitingthe scope thereof.

LIST OF REFERENCE NUMERALS

-   100 Vertical Cavity surface Emitting Laser-   105 first electrical contact-   110 substrate-   115 first distributed Bragg reflector-   116 high refractive index layer-   117 low refractive index layer-   118 Al_(y)Ga_((1-y))As-sub-layer-   119 oxidation control layer-   120 active layer-   122 current aperture-   122 a first current aperture-   122 b second current aperture-   125 current aperture layer-   125 a first current aperture layer-   125 b second current aperture layer-   125 c third current aperture layer-   125 d fourth current aperture layer-   126 oxidation profile-   127 waistline-   130 second distributed Bragg reflector-   135 second electrical contact-   150 cooling structure-   200 AlAs-content-   210 direction across VCSEL along emission direction-   250 standing wave pattern-   300 laser device-   310 electrical driving circuit-   320 electrical power supply-   330 laser array-   410 step of providing a first electrical contact,-   420 step of providing a substrate,-   430 step of providing a first distributed Bragg reflector-   440 step of providing an active layer-   450 step of providing a second distributed Bragg reflector-   460 step of providing a second electrical contact-   470 step of providing Al_(y)Ga_((1-y))As-layer

1. A Vertical Cavity Surface Emitting Laser comprising: a firstelectrical contact; a substrate; a first Distributed Bragg Reflector; anactive layer; a second Distributed Bragg Reflector; and a secondelectrical contact, wherein the Vertical Cavity Surface Emitting Lasercomprises at least two current aperture layers arranged below or abovethe active layer, wherein each of the current aperture layers comprisesa Al_(y)Ga_((1-y))As-layer, wherein a first current aperture layer ofthe at least two current aperture layers has a first current aperture,wherein a second current aperture layer of the at least two currentaperture layers has a second current aperture, wherein the first currentaperture layer of the at least two current aperture layers is arrangednearer to the active layer than the second current aperture layer of theat least two current aperture layers, wherein the first current apertureis larger than the second current aperture.
 2. The Vertical CavitySurface Emitting Laser according to claim 1, further comprising at leastone Al_(y)Ga_((1-y))As-layer with 0.95≤y≤1, wherein the at least oneAl_(y)Ga_((1-y))As-layer has a thickness of at least 40 nm, wherein theat least one Al_(y)Ga_((1-y))As-layer is separated into at least twosub-layers, wherein at least one oxidation control layer is disposedbetween the at least two sub-layers.
 3. The Vertical Cavity SurfaceEmitting Laser according to claim 2, wherein the at least one oxidationcontrol layer has a by a thickness between 0.7 nm and 3 nm.
 4. TheVertical Cavity Surface Emitting Laser according to claim 1, whereineach of the at least two current aperture layers comprises aAl_(y)Ga_((1-y))As-layer with one or more oxidation control layers. 5.The Vertical Cavity Surface Emitting Laser according to claim 1, whereinthe second current aperture is arranged at a distance corresponding toan integer multiple of half of the emission wavelength of the VerticalCavity Surface Emitting Laser to the active layer.
 6. The VerticalCavity Surface Emitting Laser according to claim 1, wherein the firstDistributed Bragg Reflector or the second Distributed Bragg Reflectorcomprises the at least one Al_(y)Ga_((1-y))As-layer.
 7. The VerticalCavity Surface Emitting Laser according to claim 2, wherein a materialof the oxidation control layer comprises Al_(x)Ga_((1-x))As, wherein0≤x≤0.9.
 8. The Vertical Cavity Surface Emitting Laser according toclaim 2, wherein the at least one Al_(y)Ga_((1-y))As-layer has y>0.99,and wherein the at least one Al_(y)Ga_((1-y))As-layer is separated by atleast two oxidation control layers, and a material of the oxidationcontrol layer comprises Al_(x)Ga_((1-x))As with 0.4≤x≤0.6.
 9. TheVertical Cavity Surface Emitting Laser according to claim 2, wherein athickness of the at least one oxidation control layer comprises between3% and 10% of a total thickness of the Al_(y)Ga_((1-y))As-layer.
 10. TheVertical Cavity Surface Emitting Laser according to claim 1, wherein atleast one of the at least one Al_(y)Ga_((1-y))As-layers comprises atapered oxidation profile, in particular wherein the current secondaperture comprises a tapered oxidation profile.
 11. The Vertical CavitySurface Emitting Laser according to claim 10, wherein the at least oneAl_(y)Ga_((1-y))As-layer with the tapered oxidation profile comprises atleast two oxidation control layers, wherein the at least two oxidationcontrol layers separate the at least one Al_(y)Ga_((1-y))As-layer in atleast three sub-layers, and wherein at least one of the three sub-layershas a different thickness as the other sub-layers.
 12. The VerticalCavity Surface Emitting Laser according to claim 10, wherein a waistlineof the tapered oxidation profile is arranged in a range of a node of astanding wave pattern of the Vertical Cavity Surface Emitting Laser whendriven at a predefined electrical driving current.
 13. The VerticalCavity Surface Emitting Laser according to claim 2, wherein the firstDistributed Bragg Reflector and the second Distributed Bragg Reflectorcomprise a multitude of high refractive index layers and a multitude oflow refractive index layers, wherein the low refractive index layerscomprise said Al_(y)Ga_((1-y))As-layers.
 14. The laser device comprisingat least one Vertical Cavity Surface Emitting Laser according to claim 1and an electrical driving circuit for electrically driving the VerticalCavity Surface Emitting Laser.
 15. A method of fabricating a VerticalCavity Surface Emitting Laser, the method comprising: providing a firstelectrical contact; providing a substrate; providing a first distributedBragg reflector; providing an active layer; providing a seconddistributed Bragg reflector; providing a second electrical contact;providing at least two current aperture layers, wherein the at least twocurrent aperture layers are arranged below or above the active layer;arranging a first current aperture layer of the at least two currentaperture layers nearer to the active layer as a second current aperturelayer the at least two current aperture layers; providing a firstcurrent aperture in the first current aperture layer; and providing asecond current aperture in the second current aperture layer with asmaller size as the first current aperture.
 16. The method offabricating a Vertical Cavity Surface Emitting Laser according to claim15, wherein each of the current aperture layers comprises at least oneAlyGa(1-y)As-layer.
 17. The method of fabricating a Vertical CavitySurface Emitting Laser according to claim 16, wherein the at least oneAlyGa(1-y)As-layer has a thickness of at least 40 nm, wherein the atleast one AlyGa(1-y)As-layer is separated into at least two sub-layers,wherein at least one oxidation control layer is disposed between the atleast two sub-layers.
 18. The method of fabricating a Vertical CavitySurface Emitting Laser according to claim 17, wherein the at least oneoxidation control layer has a by a thickness between 0.7 nm and 3 nm.19. The method of fabricating a Vertical Cavity Surface Emitting Laseraccording to claim 16, The Vertical Cavity Surface Emitting Laseraccording to claim 1, wherein each of the first current aperture layerand the second current aperture layer comprises a AlyGa(1-y)As-layerwith one or more oxidation control layers.
 20. The method of fabricatinga Vertical Cavity Surface Emitting Laser according to claim 15, whereinthe second current aperture is arranged at a distance corresponding toan integer multiple of half of the emission wavelength of the VerticalCavity Surface Emitting Laser to the active layer.